Dimerization of terminal alkynes catalyzed by chloro(η 5-pentadienyl) bis(triphenylphosphine)ruthenium(II) and kinetics of phosphine substitution

Abstract

Exchange of PMe 2Ph for PPh 3 in (η 5-pentadienyl)ruthenium{bis(triphenylphosphine)}chloride, (η 5-C 5H 7)Ru(PPh 3) 2Cl ( 1) under first order conditions proceeds rapidly in THF at room temperature. A pseudo-first order rate constant of 17 ± 2 × 10 −4 s −1 is obtained for the reaction at 21 °C. The rate constant is essentially independent of the phosphine concentration. The activation parameters, Δ H † = 16.1 ± 0.4 kcal mol −1 and Δ S † = −16 ± 1 cal K −1 mol −1 differ from those reported for phosphine exchange in CpRu(PPh 3) 2Cl ( 2) and (η 5-indenyl)Ru(PPh 3) 2Cl ( 3). The reaction of 1 with PMe 2Ph is about 70 times faster than the reaction of 2 at 30 °C and some 40 times faster than the reaction of 3 at 20 °C. (η 5-C 5H 7)Ru(PPh 3) 2Cl( 1) is more active than the ruthenium(II) complexes 2, 3, and TpRu(PPh 3) 2Cl ( 4) in the catalytic dimerization of terminal alkynes with nearly quantitative conversion of PhCCH and FcCCH at ambient temperature in 24 h. The enhanced substitution rate is accompanied by >50% conversion of phenylacetylene to oligomeric products. Reaction of 1 with NaPF 6 in acetonitrile yields the cationic ruthenium(II) complex [(η 5-C 5H 7)Ru(PPh 3) 2(CH 3CN)][PF 6] ( 7). The latter complex is much less active in reactions with phenylacetylene than 1 but avoids the formation of oligomeric products.